Fine structural analysis of neuronal connections within the cochlear nucleus will be achieved by correlating light and electron microscopy of normal and experimental cats. The region to be studied, called the "octopus cell area" (OCA) is unusual because (1) it contains only one morphological type of nerve cell that (2) receives a predominant input from one source, the cochlea. The OCA can be physically isolated from the perfusion-fixed brain for cytological study of identifiable octopus cells and their inputs. Distribution of specific synaptic inputs upon specific parts of the octopus cell body and all of its processes will be determined. Serial thin sections of normal electron microscopic preparations will resolve whether single auditory fibers can give rise to two cytologically different synaptic terminals upon octopus cells. Later, neurocytochemical studies will help determine the neurotransmitter substance within these and other endings. Experimental electron microscopic preparations following small lesions of the cochlea will be used to find any ordered pattern of cochlear terminals upon octopus cells. Other synaptic inputs from outside (extrinsic) and inside (intrinsic) the cochlear nucleus will be identified in normal and experimental animals. Distribution of extrinsic and intrinsic inputs will be shown light microscopically with silver stains for degenerating axons following lesions of other auditory nuclei or other parts of the cochlear nucleus, respectively. Later, degenerating synaptic terminals will be localized in electron micrographs following the same types of lesions. This morphological study will provide the structural basis for functional analysis of the mammalian auditory system. It may also reveal new possibilities of synaptic organization in the mammalian central nervous system.